The present invention relates to apparatus and methods for processing substrates such as semiconductor substrates for use in IC fabrication or glass panels for use in flat panel display applications. More particularly, the present invention relates to improved plasma processing systems that are capable of processing substrates with a high degree of processing uniformity across the substrate surface.
Plasma processing systems have been around for some time. Over the years, plasma processing systems utilizing inductively coupled plasma sources, electron cyclotron resonance (ECR) sources, capacitive sources, and the like, have been introduced and employed to various degrees to process semiconductor substrates and glass panels.
During processing, multiple deposition and/or etching steps are typically employed. During deposition, materials are deposited onto a substrate surface (such as the surface of a glass panel or a wafer). For example, deposited layers such as various forms of silicon, silicon dioxide, silicon nitride, metals and the like may be formed on the surface of the substrate. Conversely, etching may be employed to selectively remove materials from predefined areas on the substrate surface. For example, etched features such as vias, contacts, or trenches may be formed in the layers of the substrate.
One particular method of plasma processing uses an inductive source to generate the plasma. FIG. 1 illustrates a prior art inductive plasma processing reactor 100 that is used for plasma processing. A typical inductive plasma processing reactor includes a chamber 102 with an antenna or inductive coil 104 disposed above a dielectric window 106. Typically, antenna 104 is operatively coupled to a first RF power source 108. Furthermore, a gas port 110 is provided within chamber 102 that is arranged for releasing gaseous source materials, e.g., the etchant source gases, into the RF-induced plasma region between dielectric window 106 and a substrate 112. Substrate 112 is introduced into chamber 102 and disposed on a chuck 114, which generally acts as an electrode and is operatively coupled to a second RF power source 116.
In order to create a plasma, a process gas is input into chamber 102 through gas port 110. Power is then supplied to inductive coil 104 using first RF power source 108. The supplied RF energy couples through the dielectric window 106 and a large electric field is induced inside chamber 102. More specifically, in response to the electric field, a circulating current is induced in chamber 102. The electric field accelerates the small number of electrons present inside the chamber causing them to collide with the gas molecules of the process gas. These collisions result in ionization and initiation of a discharge or plasma 118. As is well known in the art, the neutral gas molecules of the process gas when subjected to these strong electric fields lose electrons, and leave behind positively charged ions. As a result, positively charged ions, negatively charged electrons and neutral gas molecules (and/or atoms) are contained inside the plasma 118. As soon as the creation rate of free electrons exceeds their loss rate, the plasma ignites.
Once the plasma has been formed, neutral gas molecules inside the plasma tend to be directed towards the surface of the substrate. By way of example, one of the mechanism contributing to the presence of the neutrals gas molecules at the substrate may be diffusion (i.e., the random movement of molecules inside the chamber). Thus, a layer of neutral species (e.g., neutral gas molecules) may typically be found along the surface of substrate 112. Correspondingly, when bottom electrode 114 is powered, ions tend to accelerate towards the substrate where they, in combination with neutral species, activate the etching reaction.
One problem that has been encountered with inductive plasma systems, such as the one mentioned above, has been variations in the etch performance across the substrate, e.g., a non-uniform etch rate. That is, one area of the substrate gets etched differently than another area. As a result, it is extremely difficult to control the parameters associated with the integrated circuit, i.e., critical dimensions, aspect ratios, and the like. Additionally, a non-uniform etch rate may lead to device failure in the semiconductor circuit, which typically translates into higher costs for the manufacturer. Moreover, there also exist other issues of concern such as the overall etch rate, etch profile, micro-loading, selectivity, and the like.
In recent years, it has been found that these non-uniform etch rates may be the result of variations in the plasma density across the surface of the substrate, i.e., a plasma that has regions with greater or lesser amounts of reactive species (e.g., positively charged ions). While not wishing to be bound by theory, it is believed that the variations in plasma density are created by asymmetries that are found in the power transmission characteristics of the power coupling, e.g., antenna, the dielectric window, and/or plasma. If the power coupling is asymmetric, it stands to reason that the circulating current of the induced electric field will be asymmetric, and therefore the ionization and initiation of the plasma will be asymmetric. As a result, variations in the plasma density will be encountered. For example, some antenna arrangements induce a current that is strong in the center of the coil, and weak at the outer diameter of the coil. Correspondingly, the plasma tends to congregate towards the center of the process chamber (as shown in FIG. 1 by plasma 118).
The standard technique for overcoming an asymmetric power coupling has been to compensate or balance out the asymmetries. For example, using a pair of planar antennas to increase the current density at weak current areas, joining radial members to a spiral antenna to form more circular loops at different radii, varying the thickness of the dielectric window to decrease the current density at strong current areas. However, these balancing techniques tend not to provide an azimuthally symmetric power coupling. That is, they still tend to have azithmuthal variations that lead to variations in the plasma, which makes it difficult to obtain etch uniformity.
Moreover, most antenna arrangements used today form some type of capacitive coupling between the antenna and the plasma. Capacitive coupling is created by a voltage drop between the antenna and the plasma. The voltage drop typically forms a sheath voltage at or near the coupling window. For the most part, the sheath voltage tends to act like the bottom electrode (powered). That is, the ions in the plasma tend to be accelerated across the sheath, and therefore accelerate towards the negatively charged coupling window. As a result, the accelerating ions tend to bombard the surface of the coupling window.
These bombarding ions will have substantially the same effect on the coupling window as they do on the substrate, i.e., they will either etch or deposit material on the coupling window surface. This may produce undesirable and/or unpredictable results. For example, deposited material may accumulate on the coupling window and become the source of harmful particulate, especially when material flakes off onto the substrate surface. Removing material from the coupling window will have a similar effect. Eventually, the increase or decrease in thickness will cause process variation, for example, in the power transmission properties of the power coupling (e.g., antenna, dielectric window, plasma). As mentioned, process variation may lead to non-uniform processing, which lead to device failure in the semiconductor circuit.
In view of the foregoing, there are desired improved methods and apparatuses for producing uniform processing at the surface of the substrate.